1. Field of the Invention
The present invention relates to a decryption processing apparatus for applying the decrypting process to encrypted data and a decryption method and the like, and particularly the invention relates to a technology for preventing the decryption process in advance when the encrypted data is not correctly decrypted.
2. Description of the Related Art
Recently, there has been used an encryption technology for preventing leakage and abuse of data. The encrypted data can be restored when decrypted using the correct decryption key and decryption algorithm, but the data is not correctly restored when decrypted using the wrong decryption key and algorithm.
As a method for determining whether the encrypted data is correctly restored, there is a method that directly checks the decrypted data, but it is less efficient when checking a large amount of data. Thus, there has been proposed a method that encrypts the decryption verification code and the original data at the same time, checks the decryption verification code after decrypting them to determine whether the decryption process is normally executed (for example, see Patent Document 1).
[Patent Document]                Tokkai 2003-32248 (Japanese Non-Examined Patent Publication)        
The method disclosed in the above Patent Document 1 verifies whether the decryption is correctly done by the decryption verification code after decrypting both of the encrypted decryption verification code and the original data. Thus, the decryption process is executed to the entire encrypted data also in the case where the decryption is not correctly done, so that the process load is large.
Further, when the decryption is not correctly done, data that is not correctly restored is generated. Even though the wrong data is generated as described above, the data is not normally operates when it is a program, and the abuse of data does not occur. Also, in the case of image data of a specific file format in which the data is seen through a browsing application, the data that is not correctly restored cannot be browsed because the data does not conform to the rule the application defines. Thus, even though the data that is not correctly decrypted is generated, this hardly causes the leakage and abuse of information.
However, when the encrypted data which is image data of bitmap type and the like is not correctly decrypted due to the use of a wrong decryption key, there is a high possibility that what contents the original image has is guessed from the image that is not correctly decrypted, because of the nature of the data. Also, there is a possibility that the original image is guessed with high accuracy from even the abnormal image that is not correctly decrypted, as the decryption and image output are repeated several times by changing the decryption key and some sort of regularity (such as the decryption rule of image data due to the change in the decryption key) is found. Further, the image that is not correctly decrypted is generated on a memory from which the information leakage may occur, although the image is not formed on a recording paper or other materials.